Morphology discrimination may be used in conjunction with cardiac therapy to distinguish one type of cardiac signal from another. For example, a heartbeat waveform may have a particular shape when the heart is beating in a normal manner and different shape when the heart is beating in an irregular manner (e.g., during arrhythmia). Accordingly, a cardiac device may acquire electrocardiogram signals representative of cardiac waveforms over time and use morphology discrimination to determine whether the shapes of the acquired signals indicate a regular or irregular heartbeat.
In general, a conventional morphology discrimination scheme may involve generating a template corresponding to how a given set of data is expected to appear and comparing a candidate set of data with the template to characterize the candidate set of data. For example, a template may consist of a mean and an associated standard deviation derived from several sets of data that are known or expected to be representative of some condition. Then, when it is desired to determine whether a candidate set of data also is associated with that condition, the candidate set of data is compared with the template. For example, the candidate set of data may be cross correlated with the template to determine whether this set of data falls within a range of values defined by the template.
Some conventional cardiac morphology discrimination schemes discriminate between different cardiac signals by comparing the peaks of the cardiac signals. Here, a cardiac device may generate a morphology template by collecting information regarding the magnitude (e.g., relative to a baseline) of the peaks of a set of QRS complexes that are known or expected to correspond to a given cardiac condition. At some later point in time, the device may compare the peaks of current QRS complexes with the morphology template to categorize the patient's current condition.
As a specific example, cardiac signals associated with ventricular tachycardia may be distinguished by their shape from cardiac signals associated with other arrhythmias that resemble normal sinus rhythm (e.g., super-ventricular tachycardia). In such a case, the morphology template may be defined to represent cardiac signals as they appear during normal sinus rhythm. The morphology discrimination process may then determine whether a given set of cardiac signals are associated with ventricular tachycardia or these other arrhythmias by comparing the cardiac signals to the template.
In general, morphology discrimination schemes such as these may rely on an assumption that the template information is relatively constant. Hence, the accuracy of such schemes may depend on the degree to which the signals used to generate the template fluctuate and on the sensitivity of any cardiac device component that is used to generate the electrocardiogram. For example, if a cardiac signal has peak values (either local maxima or local minima) that are close to the baseline (e.g., zero), small changes in the baseline value may affect the number of peaks detected. As a result, a measure of similarity that depends on this comparison may find quite different signals whose shapes should be considered very similar, if not identical. Accordingly, a need exists for more effective techniques for discriminating between cardiac signals.